The influence of hemispheric asymmetry and realistic basic states on tropical stationary waves in a shallow water model

A shallow water model is used to study the stationary waves in the tropical upper troposphere. Realistic zonal-mean winds are generated by imposing a zonally-symmetric topography distribution underneath a thin fluid layer and relaxing the fluid towards its global-mean depth. Basic states with zero mean meridional flow are also constructed by balancing the height field with the equilibrium zonal-mean zonal winds. Both hemispherically-symmetric (equinoctial) and hemispherically-asymmetric (solstitial) basic states are considered. Stationary waves are generated by adding a mass source-sink distribution along or near the equator.;Westerly zonal-mean flow in the subtropics amplifies the stationary wave response to tropical eddy forcing and shifts the eddy height and vorticity maxima to the east, bringing the simulated eddies into better agreement with the observed seasonally varying eddy circulations in the tropical upper troposphere. Moving the wave forcing off the equator amplifies the response in the forced hemisphere, but the response in the opposite hemisphere decreases only slightly because the eddy divergent winds act over a wide latitudinal range. The zonal-mean circulation in the solstitial basic state enhances the eddy response in the winter hemisphere and limits the response in the summer hemisphere. Hemispheric asymmetry in either the eddy forcing or the basic state also leads to cross-equatorial eddy momentum fluxes. Linear experiments exhibit stronger subtropical anomalies, weaker variations along the equator, and less hemispheric symmetry than nonlinear integrations.;When the eddy forcing is located in the summer hemisphere of the solstitial basic state, the mean meridional winds enhance the propagation of wave activity across the equator, leading to stronger cross-equatorial eddy momentum fluxes and an eddy response with similar amplitudes in both hemispheres. Hence, the anti-correlation between the mean meridional flow and eddy momentum fluxes over the equator and the striking hemispheric symmetry of the tropical stationary waves over the course of the seasonal cycle can both be attributed to the tendency for the maximum eddy and zonal-mean diabatic forcing to occur in the same latitude band. The influence of the mean meridional flow on eddy momentum fluxes at low latitudes is also demonstrated in a simple linear barotropic model.